The present invention relates to an unsaturated polyester resin composition suitable for pressure molding.
In recent years, unsaturated polyester resin compositions containing a reinforcing material such as glass fibers are used very widely in the field of electrical insulation, construction materials, etc. Such the compositions can provide cured articles having good mechanical strength, dimensional stability and electrical properties.
When the compositions are applied to the transfer or injection molding of inserts, these compositions have such problems that because of the orientation of glass fibers there occurs the separation between the resin and glass fibers during kneading and molding the composition.
There have been known a variety of resin compositions for use in structure materials, which comprise powdered fillers such as silica powder, clay, silica sand, calcium carbonate powder, etc.
U.S. Pat. No. 3,562,201 discloses in Example 3 a composition comprising an unsaturated polyester resin containing a catalyst (BPO), separators (zinc stearate and stearine), a polymerization inhibitor (hydroquinone), polystyrence powder, precipitated calcium carbonate powder, ground glass fiber, and an absorbent (very fine powder). The main advantage of the composition mentioned above is that by virtue of the absorption of liquid components of the resin by the absorbent, the composition has a reduced flow property under the normal condition and when a molding pressure is applied to the composition, the absorbed components flow out so that the flow properties of the composition are improved. The compositions described therein are useful for injection or extrusion molding, according to the patent specification.
U.S. Pat. No. 3,758,799 discloses a resin-filler composition comprising a filler which passes a 40 mesh sieve and is retained on a 100 mesh sieve. The composition comprises 65 to 80% by weight of the filler and 35 to 20% by weight of a resin. The composition may contain a glass fiber.
U.S. Pat. No. 3,658,750 discloses a thermosetting resin composition comprising a thermosetting resin, a coarse powder having a particle size of from 100 to 850 .mu.m, and a fine powder having a particle size less than 60 .mu.m. The composition has improved flow properties under pressure.
U.S. Pat. No. 3,763,080 discloses a moldable composition comprising an unsaturated polyester resin, a filler of 30 to 100 mesh and a separation inhibitor. This composition may contain a glass fiber. The specification says that since this composition has improved flow properties, it can flow into small cavities or interstices.
U.S. Pat. No. 2,890,914 discloses an electric machine such as motor, a stator of which is molded with a composition comprising an unsaturated polyester resin, a filler such as a powder of mica, silica or alumina, and glass fiber.
As disclosed in the U.S. Pat. No. 3,562,201, there have been known the compositions containing a powder of a thermoplastic resin in order to reduce the shrinkage of the composition upon the curing of it.
According to the investigations into the known compositions, it has been found that they have unsatisfactory molding characteristics when applied to transfer or injection molding of small sized devices.
When a composition contains only very fine fillers as disclosed in the U.S. Pat. No. 3,562,201, the viscosity of the composition becomes so high that a high molding pressure is needed. When the small sized devices include friable or fragile parts, the high pressure molding is not desirable.
On the other hand, when a resin composition contains only a coarse filler as disclosed in the U.S. Pat. No. 3,758,799, very fine cavities of the devices will not be sufficiently filled with the composition.
Although the U.S. Pat. No. 3,658,750 discloses a filler combination of a coarse filler and a fine filler, it is silent about fiber reinforcement. Further, in the patent specification only hard materials as a fine filler are exemplified. It has been found that when the hard fine filler is used, a glass fiber is ground and shortened so that the function for reinforcement by the glass fiber is lost.
The composition disclosed in the U.S. Pat. No. 3,763,080 contains only a coarse filler, which has the above mentioned drawback.